how to declare 2D vector

[luckycusp]

I want to define a 3x4 2D vector, how can I do it?

For 1D vector, I can do

std::vector<double> a(3);

but for 2D?

[akmal4ik]

it's impossible, because vector is a dynamic array, but not matrix(2D array).

[Warnis]

Sure its possible:
std::vector<std::vector<double> > array;

[luckycusp]

it's impossible, because vector is a dynamic array, but not matrix(2D array).

If we can have a contructor for 1D array, there could also be a for a 2D array.

Sure its possible:
std::vector<std::vector<double> > array;

Thanks but i want to declare it for specific size(say 3*4), and not keep doing 'push_back's latter.

[Warnis]

Well then its impossible, unless you use a 1D vector and have it simulate a 2D array

[luckycusp]

Nevermind, I wrote a small function and it does it for me!

   void Change2DVectorSize(vector< vector<double> > &a, int nRows,int nColumns)
   {
	   int i;
	   vector<double> CurrRow;

	   a.clear();

	   for(i=0;i<nColumns;i++)
		   CurrRow.push_back(0.0);

	   for(i=0;i<nRows;i++)
		   a.push_back(CurrRow);	    
   }

It would have been nice if such a function was inbuilt in C++. Btw, can somebody help me change the above code from 'double' to 'template'? So that I can reuse it for other datatypes.. int etc. Let me give it a try:-

   template < typename T >
   void Change2DVectorSize(vector< vector<T> > &a, int nRows,int nColumns)
   {
	   int i;
	   vector<T> CurrRow;

	   a.clear();

	   for(i=0;i<nColumns;i++)
		   CurrRow.push_back((T)0);

	   for(i=0;i<nRows;i++)
		   a.push_back(CurrRow);	    
   }

[jsmith]

std::vector< std::vector< double > > twoDMatrix( 3, std::vector< double >( 4 ) );

done.

[luckycusp]

Jsmith

Now thats what I call to the point and perfect answer!! Thanks.

Is my use of template correct? (Am quite new to it)

[jsmith]

It is.